1. Technical Field of the Invention
The present invention relates generally to lasers and more particularly to quantification, reduction and elimination of parasitic lasing.
2. Discussion of the Related Art
The efficiency and lifetime of a high gain laser can be limited by parasitic lasing, which is unwanted lasing that can develop between any optical surfaces in the system. Polarization rotating devices such as waveplates and electro-optic components can also dramatically affect the circulating optical power of the parasitic lasing. The lasing is termed "parasitic" because it is uncontrolled lasing which depletes the gain, thereby making the laser less efficient. This unwanted stimulated emission is also important to identify and eliminate because the same optical surfaces which participate in parasitic lasing, can cause catastrophic optical damage to the laser optics. In Q-switched lasers, for example, light which retro-reflects back into the laser will typically destroy the optical coatings and surfaces.
Parasitic lasing is different from amplified spontaneous emission (ASE). Amplified spontaneous emission generally refers to amplified fluorescence inside of a laser rod or slab. The gain inside the laser medium becomes so high, that the amplified fluorescence decreases the upper laser level population via stimulated emission, thereby depleting the gain. Both parasitic lasing and ASE can greatly reduce laser efficiency, but parasitic lasing can cause optical damage, and will be spectrally bright and propagate with a small beam divergence. These features of parasitic lasing make it particularly harmful for lidar applications. Several orders of magnitude of parasitic protection may be gained using this controlled-feedback technique.
Oscillator models have been developed which use the measured values of reflectivity, the system's small-signal-gain, passive and active losses, and resonator configuration to quantify parasitic lasing. This technique is primarily useful for parasitic lasing along the optical axis of a laser system; ASE or parasitic lasing transverse to the optic axis can potentially be identified but not fully characterized using this retroreflection technique. Previous work on parasitic lasing has focused on parasitic lasing only within the laser crystal, typically a Neodymium doped disk or slab.
It is accordingly an object of the present invention to provide a method for quantifying parasitic lasing.
It is a further object of the present invention to provide a method for eliminating parasitic lasing.
It is yet another object of the present invention to accomplish the foregoing objects in a simple manner.
Additional objects and advantages of the present invention are apparent from the drawings and specification that follow.